Dual-comb interferometry used for instance in spectroscopy has attracted a lot of attention recently, as reported in the review article entitled “Dual-comb spectroscopy” authored by Coddington, I., Newbury, N. & Swann, W. and published in Optica (2016).
Generally put, a comb source refers to a laser source being configured to emit an optical field with a periodic amplitude pattern having an optical comb spectrum or a frequency comb, i.e. an optical spectrum having a plurality of discrete and equally spaced optical spectral components. The periodic amplitude pattern can take the form of a laser pulse train.
As depicted in FIG. 1A, dual-comb interferometry involves a dual-comb source or two comb sources 10a,10b emitting an optical signal 13 including two mutually coherent optical fields 11a,11b with slightly detuned repetition rates and respective optical comb spectrums 12a,12b. A volume 14 can be interrogated by either or both of the optical fields 11a,11b as shown by the two possible locations of the volume 14. The two optical fields 11a,11b are interfered with one another on an optical detector 15 to generate a beat note, typically referred to as a beating interference signal 16, with spectral components in the radio frequency (rf) domain. The beating interference signal 16 is typically acquisitioned using an acquisition unit 17.
When the two comb sources 10a,10b are mutually coherent or referenced to one another, for instance using a phase locking system 18, the beating interference signal 16 exhibits an electrical comb spectrum 19 which can be mapped to an optical comb spectrum of the optical signal in the optical frequency (of) domain. Accordingly, interferometric measurements can be performed on the optical signal via the downsized, and measurable, electrical signal.
The stability of the two comb sources 10a,10b relative to one another is required to ensure that the frequency components of the beating interference signal are consistent during the whole time interval of the acquisition by the acquisition unit 17. It is known that even minor repetition rate and/or carrier frequency variations of any of the comb sources 10a,10b can change the mapping between the electrical signal and the optical signal, thus limiting the resulting spectral resolution.
There exists a number of techniques to phase-lock or reference two comb sources 10a,10b to one another. For instance, an exemplary phase locking system 18 is presented in FIG. 1B. However, as can be understood from FIG. 1B, these techniques typically involve additional hardware, and thus additional costs, which limits the widespread use of dual-comb interferometry in some applications. There thus remains room for improvement.